In the analysis and design of steel frames, there are various factors that the uncertainty of each of them can have a significant effect on structural safety. Hence, the investigation and recognition of these factors is important for understanding the true behavior of the structure. In the meantime, the stiffness beam-to-column connection can be considered as one of the factors due to uncertainties such as how to construct, execute, install and model the parts and joints steel structures. The effect of semi-rigid joint on structural analysis not only changes the moment distribution along the beams and columns, but also increases the frame's displacement due to second-order effects. Therefore, in most advanced specifications, it is allowed to consider semi-rigid joints behavior. Hence, in this paper, first, the effect of stiffness of the connections in the plastic analysis based on plastic hinge will be studied in two steel frames. In the following, by randomly assuming the stiffness of rotation of beam to column and stiffness of column joints to the support and plastic moment of members and joints, and considering several probability distribution with different coefficients of variant, the behavior of the structure in the probabilistic plastic analysis can be investigated. Accordingly, the Monte-Carlo simulation is used to estimate the probability of failure of semi-rigid steel frames. On this Basis, each frame is simulated 840000 times and the load failure capacity on the structure is available. It should be added, the OpenSees software was used to deterministic and probabilistic analysis. The results showed that the rotational stiffness of the joints and restraints and plastic moment of members had a significant effect on the responses of deterministic and probabilistic analysis.